The only advice I would give you is this:
Do not tell him you know about the affair
Tell him you are leaving him because being married to him is miserable, you don’t love him anymore, you don’t fancy him anymore, you are sick of his controlling and abusive ways etc. Tell him that this week without him around made you realise that you are happier without him in your life.
It will sting his ego. It gives you power.
And honestly it’s the truth - you wanted to divorce him before you found out about the OW, that was just the icing on the cake.
If he thinks you left him because you found out he is cheating….. he will still be waltzing around thinking he’s some fucking prize to women. I bet it will keep him awake at night stewing - that in the end, he just wasn’t good enough for you! That you dumped his sorry arse because he was a terrible husband.
Keep the fact you know about the affair quiet as long as possible. Make him think you found out after you left him.